1. Field of the Invention
This invention relates to a computerized tomography (CT) system and more particularly, this invention relates to an improvement of an algorithm for reconstructing a tomogram.
2. Description of the Prior Art
FIG. 1 is a drawing of the constitution of a detector unit of a conventional CT system to perform the computerized tomography (CT). In the figure, 1 represents a radiation source such as X-ray source, 2.sub.1 -2.sub.m represent radiation detectors such as X-ray detectors, 2 represents a radiation detector array composed by radiation detectors 2.sub.1 -2.sub.m, and 3 represents an imaging model. The radiation source 1 and the radiation detector array 2 collectively constituting a detector unit 4, the both being positioned to be interposed by the imaging model 3, are rotated and driven around the imaging model 3 with a driving unit not shown with the fixed relative positions to each other.
Operations are now explained. First, the radiation detectors 2.sub.1 -2.sub.m detect radiation beams irradiated from the radiation source 1. The radiation beam emitted from the radiation source 1 transmits each part of the imaging model 3, to project to the radiation detectors 2.sub.1 -2.sub.m at a radiation dose being attenuated by the absorption of the imaging model 3.
The radiation source 1 and the radiation detectors 2.sub.1 -2.sub.m which constitute the detector units 4 mechanically integrated, gradually rotate around the imaging model 3 with the fixed relative positions to each other, interposing the imaging model 3 between them. Then, at each rotary position, the radiation source 1 projects radiation beams and then, detected signals of radiation which are detected by each radiation detector of 2.sub.1 -2.sub.m, are measured, respectively.
Thus, the function of this system is to determine the distribution image of the radiation absorption power of each part of the imaging model 3 which is calculated from detected signals on all rotary positions and to display it as a cross-sectional image on CRT. The detected data of the radiation detectors 2.sub.1 -2.sub.m which transmit the imaging model 3 at a rotary position correspond to the fan-shaped distribution of the radiation beam and therefore, the converted data from the fan-beam projection data to parallel-beam projection data are required in order to calculate the distribution of radiation absorption power. Hence, as is conventional, data corresponding to parallel projection beam are calculated by interpolation from the fan-beam projection data and then, the distribution image of the radiation absorption power is reconstructed using an algorithm for image reconstruction such as filtered back projection method and the like.
Since the conventional computerized tomography system has been constituted in the way as described above, a large number of calculations for the interpolation have caused problems such as requirement of long time for the calculations. In addition, the calculations for interpolation have caused problems such that the spatial resolution of the absorption-power distribution image obtained as a result, gets deteriorated and the reconstructed image is of poor sharpness.